1. Field of the Invention
The present invention relates to an integrated circuit for automobiles. More particularly, the present invention relates to an integrated circuit which controls the loads of electric apparatuses such as motors and lamps of an automobile by judging the "on/off" status of a control signal for switching the loads.
2. Description of the Prior Art
An integrated circuit for automobiles has been previously employed which operates, for example, to control the movement of a windshield wiper by judging an "on/off" status of a signal from the switch is illustrated in FIG. 1. As shown in FIG. 1, the integrated circuit (10) judges the "on/off" status of a signal from the switch (20) so as to supply a control signal to be inputted to a driver circuit for driving the windshield wipers (not shown). The integrated circuit (10) comprises a C-MOS inverter (13) which consists of a p-channel type MOSFET (11) and an n-channel type MOSFET (12), a couple of diodes (14) and (15) which regulate the input voltage so that it is within a predetermined range so as to protect the aforesaid C-MOS inverter (13), a protection resistor (19), a D flip-flop (16) which functions as an input register for the driver circuit, an inverter (17) and an input latching pulse generating circuit (21) for providing a latch pulse to inverter (17).
In the circuit of FIG. 1, a pull-up resistor (30) is also employed to adjust the voltage of the "on/off" signal from switch (20) in order to match it to the level which can be input to the integrated circuit (10). Since the pull-up resistor (30) is required to continuously pass the current I.sub.1 while the switch (20) is "on", it cannot be connected to the regulated power supply (40) "Vcc" of the integrated circuit (10), for power supply (40) does not have enough capacity. Pull-up resistor (30) instead should be directly connected to the battery (B) of the automobile or indirectly connected to the battery through the ignition switch (not shown). Finally, between the input electrode (18) of the integrated circuit (10) and the pull-up resistor (30), a surge protection circuit (50) comprised of a diode (51), a capacitor (52) and a resistor (53) with a high resistance value is inserted as shown to protect the integrated circuit from battery surges.
Hereinafter, the operating process of the prior art integrated circuit (10) of FIG. 1 will be explained with reference to the time chart of FIG. 2. The indicated voltages and currents correspond to those at the similarly labelled portions of the diagram of FIG. 1.
I: Operating process from time t1 to the time just before time t3:
At time t1, when the switch (20) is turned from "off" to "on" (FIG. 2(1)), a current "I.sub.1 " is input through the pull-up resistor (30) (FIG. 2(3)) and the signal "V1" to be input to the integrated circuit (10) falls down from a "high level" (hereafter referred to as "H") to a "low level" (hereafter referred to as "L") (FIG. 2(4)). Due to this change, the p-channel type MOSFET (11) is turned "on", and the n-channel type MOSFET (12) is turned "off". The signal for the input "D" of D flip-flop (16) (V.sub.2) is raised to "H" (FIG. 2(5)). Then, at time t2, a positive input latching pulse (FIG. 2(2)) which is supplied by the input latching pulse generating circuit (21) has its polarity changed by inverter (17) and is inputted to the clock input "CLK" of D flip-flop (16). The output signal "V3" which is supplied from the output Q then falls down to "L" in synchronization with the trailing edge of the input latching pulse (FIG. 2(6)). After time t2, the signal "V3" which is supplied from the output "Q" is continuously maintained at "L".
II: Operating process from time t3 to time t4:
At time t3, the switch (20) is turned from "on" to "off" (FIG. 2(1)) and the current "I.sub.1 " which has been passing through the pull-up resistor (30) is stopped (FIG. 2(3)). The input signal "V1" also rises from "L" to "H" (FIG. 2(4)). Due to this change, the p-channel type MOSFET (11) is turned "off" and the n-channel type MOSFET (12) is turned "on", and the signal "V2" thus falls down to "L" (FIG. 2(5)). The signal "V3" from the Q output of the D flip-flop (16) is maintained at "L" (FIG. 2(6)). Then, at time t4 the signal "V3" rises up from "L" to "H" in synchronization with the trailing edge of the input latching pulse which is supplied to the clock "CLK" of D flip-flop (16) (FIGS. 2(2) and 2(6)).
As indicated above, the integrated circuit (10) in the prior art circit of FIG. 1 judges the "on/off" status of a signal from the switch (20) and accordingly operates a driver circuit (not shown) of the windshield wipers by a "L" level of the signal "V3". However, such an integrated circuit of the prior art has the following problems:
1. Due to the treatment of current leakage while the switch (20) is "off", the resistance for the pull-up resistor (30) cannot be selected to be large enough. A smaller resistance causes a larger current to pass through the aforementioned pull-up resistor (30) while the switch (20) is "on". Moreover, the effective power to be consumed through this process is much lager when the ignition key is turned "off"; therefore, the vehicle battery is frequently drained.
2. To protect the integrated circuit from surge voltage such as ignition noises or loading-dumping pulses, a surge protection circuit (50) must be used. Surge protection circuit (50) increases the production costs and occupies much space. In fact, the space for the surge protection circuit (50) is much wider than that required for the integrated circuit (10) itself.